1. Field of the Invention
The present invention relates to an optical module used in optical communications, and more specifically, to an optical module that can match the impedance of signal pins to that of signal lines even if no impedance matching circuit is provided.
2. Background Art
In recent years, increase in TV channels, such as satellite broadcasting, has been required in cable television to meet the diversity of consumers' tastes. In optical modules used in the delivery of cable television broadcasting, wider bandwidth is also required. On the other hand, since cost reduction is also required, expensive parts to meet requests for bandwidth cannot be used. Conventional optical modules have been constructed in a form referred to as TO-CAN, wherein signal pins provided in the header are simply joined to a laser diode by wire bonding, and have not been designed from the viewpoint of bandwidth widening.
FIG. 10 is a perspective view illustrating a state wherein a conventional optical module is mounted on a printed circuit board, and FIG. 11 is a side view thereof. A header 4 is provided with a lens cap 14, to which a receptacle 15 is connected. The optical module is mounted on the printed circuit board 17 so that the printed circuit board 17 is pinched between the signal pin 1 and the grounding pin 27 of the optical module. A signal line 32 and a ground conductor 31 are provided on the surface and the back face of the printed circuit board 17, respectively, each of which is connected to the signal pin 1 and the grounding pin 27, respectively by a solder 28 so as to be shortest. Since the parasitic inductance components of the signal pin 1 and the grounding pin 27 can be minimized by thus connecting, the deterioration of signals transmitted from the signal line 32 on the printed circuit board 17 can be suppressed, and the signals can be transmitted into the optical module.
However, by the above-described method for fixing the optical module to the printed circuit board, if the gap between the optical module and the printed circuit board was varied, the parasitic inductance of the signal pin 1 and the grounding pin 27 was also varied to change high-frequency characteristics, and stable production was difficult. Furthermore, for signals of higher frequencies, since the parasitic inductance of the signal pin 1 and the grounding pin 27 was measurably large to the signals, a matching circuit for the high-frequency signals was needed on the printed circuit board, and problems of increase in the number of parts and the expansion of mounting space for the matching circuit were caused. To solve these problems, an optical module using a flexible board connecting between the optical module and the printed circuit board has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 2005-286305).